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RFC 8552

Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves

Pages: 15
Group: DNSOP
BCP 222
Errata
BCP 222 is also:  8553

Top   ToC   RFC8552 - Page 1
Internet Engineering Task Force (IETF)                        D. Crocker
Request for Comments: 8552                   Brandenburg InternetWorking
BCP: 222                                                      March 2019
Category: Best Current Practice
ISSN: 2070-1721


         Scoped Interpretation of DNS Resource Records through
                "Underscored" Naming of Attribute Leaves

Abstract

   Formally, any DNS Resource Record (RR) may occur under any domain
   name.  However, some services use an operational convention for
   defining specific interpretations of an RRset by locating the records
   in a DNS branch under the parent domain to which the RRset actually
   applies.  The top of this subordinate branch is defined by a naming
   convention that uses a reserved node name, which begins with the
   underscore character (e.g., "_name").  The underscored naming
   construct defines a semantic scope for DNS record types that are
   associated with the parent domain above the underscored branch.  This
   specification explores the nature of this DNS usage and defines the
   "Underscored and Globally Scoped DNS Node Names" registry with IANA.
   The purpose of this registry is to avoid collisions resulting from
   the use of the same underscored name for different services.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8552.
Top   ToC   RFC8552 - Page 2
Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Underscore-Based Scoping  . . . . . . . . . . . . . . . .   3
     1.2.  Scaling Benefits  . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Global Underscored Node Names . . . . . . . . . . . . . .   4
     1.4.  Interaction with DNS Wildcards  . . . . . . . . . . . . .   5
     1.5.  History . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  "Underscored and Globally Scoped DNS Node Names" Registry . .   6
   3.  Guidance for Registering RRset Use  . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  "Underscored and Globally Scoped DNS Node Names" Registry   8
     4.2.  Enumservices Registrations Registry . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The core Domain Name System (DNS) technical specifications ([RFC1035]
   and [RFC2181]) assign no semantics to domain names or their parts,
   and no constraints upon which resource record (RR) types are
   permitted to be stored under particular names [RFC1035] [RFC2181].
   Over time, some leaf node names, such as "www" and "ftp", have come
   to imply support for particular services, but this is a matter of
   operational convention rather than defined protocol semantics.  This
   freedom in the basic technology has permitted a wide range of
   administrative and semantic policies to be used -- in parallel.  DNS
   data semantics have been limited to the specification of particular
   resource record types on the expectation that new resource record
Top   ToC   RFC8552 - Page 3
   types would be added as needed.  Unfortunately, the addition of new
   resource record types has proven extremely challenging, with
   significant adoption and use barriers occurring over the life of the
   DNS.

1.1.  Underscore-Based Scoping

   As an alternative to defining a new RR TYPE, some DNS service
   enhancements call for using an existing resource record type but
   specifying a restricted scope for its occurrence.  Scope is meant as
   a static property, not one dependent on the nature of the query.  It
   is an artifact of the DNS name.  That scope is a leaf node containing
   the specific resource record sets that are formally defined and
   constrained.  Specifically:

      The leaf occurs in a branch having a distinguished naming
      convention: there is a parent domain name to which the scoped data
      applies.  The branch is under this name.  The sub-branch is
      indicated by a sequence of one or more reserved DNS node names; at
      least the first (highest) of these names begins with an underscore
      (e.g., "_name").

   Because the DNS rules for a "host" (host name) do not allow use of
   the underscore character, the underscored name is distinguishable
   from all legal host names [RFC0952].  Effectively, this convention
   for naming leaf nodes creates a space for the listing of "attributes"
   -- in the form of resource record types -- that are associated with
   the parent domain above the underscored sub-branch.

   The scoping feature is particularly useful when generalized resource
   record types are used -- notably "TXT", "SRV", and "URI" [RFC1035]
   [RFC2782] [RFC6335] [RFC7553].  It provides efficient separation of
   one use of them from others.  Absent this separation, an
   undifferentiated mass of these RRsets is returned to the DNS client,
   which then must parse through the internals of the records in the
   hope of finding ones that are relevant.  Worse, in some cases, the
   results are ambiguous because a record type might not adequately
   self-identify its specific purpose.  With underscore-based scoping,
   only the relevant RRsets are returned.

   A simple example is DomainKeys Identified Mail (DKIM) [RFC6376],
   which uses "_domainkey" to define a place to hold a TXT record
   containing signing information for the parent domain.

   This specification formally defines how underscored names are used as
   "attribute" enhancements for their parent domain names.  For example,
   the domain name "_domainkey.example." acts as an attribute of the
   parent domain name "example.".  To avoid collisions resulting from
Top   ToC   RFC8552 - Page 4
   the use of the same underscored names for different applications
   using the same resource record type, this document establishes the
   "Underscored and Globally Scoped DNS Node Names" registry with IANA.
   Use of such node names, which begin with an underscore character, is
   reserved when they are the underscored name closest to the DNS root;
   as in that case, they are considered "global".  Underscored names
   that are farther down the hierarchy are handled within the scope of
   the global underscored node name.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Scaling Benefits

   Some resource record types are used in a fashion that can create
   scaling problems if an entire RRset associated with a domain name is
   aggregated in the leaf node for that name.  An increasingly popular
   approach, with excellent scaling properties, places the RRset under a
   specially named branch, which is in turn under the node name that
   would otherwise contain the RRset.  The rules for naming that branch
   define the context for interpreting the RRset.  That is, rather than:

                            domain-name.example
                              /
                             RRset

   the arrangement is:

                        _branch.domain-name.example
                          /
                         RRset

   A direct lookup to the subordinate leaf node produces only the
   desired record types, at no greater cost than a typical DNS lookup.

1.3.  Global Underscored Node Names

   As defined in [RFC1034], the DNS uses names organized in a tree-
   structured or hierarchical fashion.  A domain name might have
   multiple node names that begin with the underscore character (e.g.,
   "_name").  A global underscored node name is the one that is closest
   to the root of the DNS hierarchy, also called the highest level or
   topmost.  In the presentation convention described in Section 3.1 of
   [RFC1034], this is the rightmost name beginning with an underscore.
Top   ToC   RFC8552 - Page 5
   In other presentation environments, it might be positioned
   differently.  To avoid concern for the presentation variations, the
   qualifier "global" is used here.

1.4.  Interaction with DNS Wildcards

   DNS wildcards interact poorly with underscored names in two ways:

   Since wildcards are only interpreted as leaf names, one cannot create
   the equivalent of a wildcard name for prefixed names.  A name such as
   label.*.example.com is not a wildcard.

   Conversely, a wildcard such as *.example.com can match any name
   including an underscored name.  So, a wildcard might match an
   underscored name, returning a record that is the type controlled by
   the underscored name but is not intended to be used in the
   underscored context and does not conform to its rules.

1.5.  History

   Originally, different uses of underscored node names developed
   largely without coordination.  For TXT records, there is no
   consistent, internal syntax that permits distinguishing among the
   different uses.  In the case of the SRV RR and URI RR, distinguishing
   among different types of use was part of the design (see [RFC2782]
   and [RFC7553]).  The SRV and URI specifications serve as templates,
   defining RRs that might only be used for specific applications when
   there is an additional specification.  The template definition
   included reference to two levels of tables of names from which
   underscored names should be drawn.  The lower-level (local scope) set
   of "_service" names is defined in terms of other IANA tables, namely
   any table with symbolic names.  The upper-level (global scope) SRV
   naming field is "_proto", although its pool of names is not
   explicitly defined.

   The aggregate effect of these independent efforts was a long list of
   underscored names that were reserved without coordination, which
   invites an eventual name-assignment collision.  The remedy is this
   base document and a companion document ([RFC8553]), which define a
   registry for these names and attempt to register all those already in
   use as well as to direct changes to the pre-registry specifications
   that used global underscored node names.
Top   ToC   RFC8552 - Page 6
2.  "Underscored and Globally Scoped DNS Node Names" Registry

   A registry for global DNS node names that begin with an underscore is
   defined here.  The purpose of the "Underscored and Globally Scoped
   DNS Node Names" registry is to avoid collisions resulting from the
   use of the same underscored name for different applications.

      If a public specification calls for use of an underscored node
      name, the global underscored node name -- the underscored name
      that is closest to the DNS root -- MUST be entered into this
      registry.

   An underscored name defines the scope of use for specific resource
   record types, which are associated with the domain name that is the
   "parent" to the branch defined by the underscored name.  A given name
   defines a specific, constrained context for one or more RR TYPEs,
   where use of such record types conforms to the defined constraints.

   o  Within a leaf that is underscore scoped, other RRsets that are not
      specified as part of the scope MAY be used.

   Structurally, the registry is defined as a single, flat table of RR
   TYPEs, under node names beginning with underscore.  In some cases,
   such as for use of an SRV record, the full scoping name might be
   multi-part, as a sequence of underscored names.  Semantically, that
   sequence represents a hierarchical model, and it is theoretically
   reasonable to allow reuse of a subordinate underscored name in a
   different, global underscored context; that is, a subordinate name is
   meaningful only within the scope of the global underscored node name.
   Therefore, they are ignored by this "Underscored and Globally Scoped
   DNS Node Names" registry.  This registry is for the definition of
   highest-level -- that is, global -- underscored node name used.

                      +----------------------------+
                      |                       NAME |
                      +----------------------------+
                      |                  _service1 |
                      |          _protoB._service2 |
                      |          _protoB._service3 |
                      |          _protoC._service3 |
                      |    _useX._protoD._service4 |
                      | _protoE._region._authority |
                      +----------------------------+

                  Table 1: Examples of Underscored Names
Top   ToC   RFC8552 - Page 7
   Only global underscored node names are registered in the "Underscored
   and Globally Scoped DNS Node Names" registry.  From the example
   above, that would mean _service1, _service2, _service3, _service 4,
   and _authority would be listed in the IANA registry.

   o  The use of underscored node names is specific to each RR TYPE that
      is being scoped.  Each name defines a place but does not define
      the rules for what appears underneath that place, either as
      additional underscored naming or as a leaf node with resource
      records.  Details for those rules are provided by specifications
      for individual RR TYPEs.  The sections below describe the way that
      existing underscored names are used with the RR TYPEs that they
      name.

   o  Definition and registration of subordinate underscored node names
      are the responsibility of the specification that creates the
      global underscored node name registry entry.

   That is, if a scheme using a global underscored node name has one or
   more subordinate levels of underscored node naming, the namespaces
   from which names for those lower levels are chosen are controlled by
   the parent underscored node name.  Each registered global underscored
   node name owns a distinct, subordinate namespace.

3.  Guidance for Registering RRset Use

   This section provides guidance for specification writers, with a
   basic template they can use, to register new entries in the
   "Underscored and Globally Scoped DNS Node Names" registry.  The text
   can be added to specifications using RR TYPE / _NODE NAME
   combinations that have not already been registered:

      Per RFC 8552, please add the following entry to the "Underscored
      and Globally Scoped DNS Node Names" registry:

   +---------+-------------------+-------------------------------------+
   | RR Type | _NODE NAME        | Reference                           |
   +---------+-------------------+-------------------------------------+
   | {RR     | _{DNS global node | {citation for the document making   |
   | TYPE}   | name}             | the addition.}                      |
   +---------+-------------------+-------------------------------------+

                   Table 2: Template for Entries in the
         "Underscored and Globally Scoped DNS Node Names" Registry
Top   ToC   RFC8552 - Page 8
4.  IANA Considerations

   IANA has established the "Underscored and Globally Scoped DNS Node
   Names" registry.  This section describes the registry, the
   definitions, the initial entries, the use of_ta and _example, and the
   use of [RFC8126] as guidance for expert review.  IANA has also
   updated the "Enumservices Registrations" registry with a pointer to
   this document.

4.1.  "Underscored and Globally Scoped DNS Node Names" Registry

   The "Underscored and Globally Scoped DNS Node Names" registry
   includes any DNS node name that begins with the underscore character
   ("_", ASCII 0x5F) and is the underscored node name closest to the
   root; that is, it defines the highest level of a DNS branch under a
   "parent" domain name.

   o  This registry operates under the IANA rules for "Expert Review"
      registration; see Section 4.1.5.

   o  The contents of each entry in the registry are defined in
      Section 4.1.1.

   o  Each entry in the registry MUST contain values for all of the
      fields specified in Section 4.1.1.

   o  Within the registry, the combination of RR Type and _NODE NAME
      MUST be unique.

   o  The table is to be maintained with entries sorted by the first
      column (RR Type) and, within that, the second column (_NODE NAME).

   o  The required Reference for an entry MUST have a stable resolution
      to the organization controlling that registry entry.
Top   ToC   RFC8552 - Page 9
4.1.1.  Contents of an Entry in the "Underscored and Globally Scoped DNS
        Node Names" Registry

   A registry entry contains:

      RR Type:    Lists an RR TYPE that is defined for use within this
                  scope.

      _NODE NAME: Specifies a single, underscored name that defines a
                  reserved name; this name is the global entry name for
                  the scoped resource record types that are associated
                  with that name.  For characters in the name that have
                  an uppercase form and a lowercase form, the character
                  MUST be recorded as lowercase to simplify name
                  comparisons.

      Reference:  Lists the specification that defines a record type and
                  its use under this _Node Name.  The organization
                  producing the specification retains control over the
                  registry entry for the _Node Name.

   Each RR TYPE that is to be used with a _Node Name MUST have a
   separate registry entry.

4.1.2.  Initial Node Names

   The initial entries in the registry are as follows:

          +------------+-----------------------+---------------+
          | RR Type    | _NODE NAME            | Reference     |
          +------------+-----------------------+---------------+
          | *          | _example              | Section 4.1.4 |
          | NULL       | _ta-* {Section 4.1.3} | [RFC8145]     |
          | OPENPGPKEY | _openpgpkey           | [RFC7929]     |
          | SMIMEA     | _smimecert            | [RFC8162]     |
          | SRV        | _dccp                 | [RFC2782]     |
          | SRV        | _http                 | [RFC4386]     |
          | SRV        | _ipv6                 | [RFC5026]     |
          | SRV        | _ldap                 | [RFC4386]     |
          | SRV        | _ocsp                 | [RFC4386]     |
          | SRV        | _sctp                 | [RFC2782]     |
          | SRV        | _sip                  | [RFC5509]     |
          | SRV        | _tcp                  | [RFC2782]     |
          | SRV        | _udp                  | [RFC2782]     |
          | SRV        | _xmpp                 | [RFC3921]     |
          | TLSA       | _dane                 | [RFC7671]     |
          | TLSA       | _sctp                 | [RFC6698]     |
          | TLSA       | _tcp                  | [RFC6698]     |
Top   ToC   RFC8552 - Page 10
          | TLSA       | _udp                  | [RFC6698]     |
          | TXT        | _acme-challenge       | [RFC8555]     |
          | TXT        | _dmarc                | [RFC7489]     |
          | TXT        | _domainkey            | [RFC6376]     |
          | TXT        | _mta-sts              | [RFC8461]     |
          | TXT        | _spf                  | [RFC7208]     |
          | TXT        | _sztp                 | [ZEROTOUCH]   |
          | TXT        | _tcp                  | [RFC6763]     |
          | TXT        | _udp                  | [RFC6763]     |
          | TXT        | _vouch                | [RFC5518]     |
          | URI        | _acct                 | [RFC6118]     |
          | URI        | _dccp                 | [RFC7566]     |
          | URI        | _email                | [RFC6118]     |
          | URI        | _ems                  | [RFC6118]     |
          | URI        | _fax                  | [RFC6118]     |
          | URI        | _ft                   | [RFC6118]     |
          | URI        | _h323                 | [RFC6118]     |
          | URI        | _iax                  | [RFC6118]     |
          | URI        | _ical-access          | [RFC6118]     |
          | URI        | _ical-sched           | [RFC6118]     |
          | URI        | _ifax                 | [RFC6118]     |
          | URI        | _im                   | [RFC6118]     |
          | URI        | _mms                  | [RFC6118]     |
          | URI        | _pres                 | [RFC6118]     |
          | URI        | _pstn                 | [RFC6118]     |
          | URI        | _sctp                 | [RFC6118]     |
          | URI        | _sip                  | [RFC6118]     |
          | URI        | _sms                  | [RFC6118]     |
          | URI        | _tcp                  | [RFC6118]     |
          | URI        | _udp                  | [RFC6118]     |
          | URI        | _unifmsg              | [RFC6118]     |
          | URI        | _vcard                | [RFC6118]     |
          | URI        | _videomsg             | [RFC6118]     |
          | URI        | _voice                | [RFC6118]     |
          | URI        | _voicemsg             | [RFC6118]     |
          | URI        | _vpim                 | [RFC6118]     |
          | URI        | _web                  | [RFC6118]     |
          | URI        | _xmpp                 | [RFC6118]     |
          +------------+-----------------------+---------------+

                     Table 3: Initial Contents of the
         "Underscored and Globally Scoped DNS Node Names" Registry

4.1.3.  _ta

   Under the NULL RR Type, the entry "_ta-*" denotes all node names
   beginning with the string "_ta-*".  It does NOT refer to a DNS
   wildcard specification.
Top   ToC   RFC8552 - Page 11
4.1.4.  _example

   The node name "_example" is reserved across all RRsets.

4.1.5.  Guidance for Expert Review

   This section provides guidance for expert review of registration
   requests in the "Underscored and Globally Scoped DNS Node Names"
   registry.

      This review is solely to determine adequacy of a requested entry
      in this registry, and it does not include review of other aspects
      of the document specifying that entry.  For example, such a
      document might also contain a definition of the resource record
      type that is referenced by the requested entry.  Any required
      review of that definition is separate from the expert review
      required here.

   The review is for the purposes of ensuring that:

   o  The details for creating the registry entry are sufficiently
      clear, precise, and complete

   o  The combination of the underscored name, under which the listed
      resource record type is used, and the resource record type is
      unique in the table

   For the purposes of this expert review, other matters of the
   specification's technical quality, adequacy, or the like are outside
   of scope.

4.2.  Enumservices Registrations Registry

   The following note has been added to the "Enumservice Registrations"
   registry:

      When adding an entry to this registry, strong consideration should
      be given to also adding an entry to the "Underscored and Globally
      Scoped DNS Node Names" registry.

5.  Security Considerations

   This memo raises no security issues.
Top   ToC   RFC8552 - Page 12
6.  References

6.1.  Normative References

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, DOI 10.17487/RFC0952,
              October 1985, <https://www.rfc-editor.org/info/rfc952>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, DOI 10.17487/RFC3921, October 2004,
              <https://www.rfc-editor.org/info/rfc3921>.

   [RFC4386]  Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
              Infrastructure Repository Locator Service", RFC 4386,
              DOI 10.17487/RFC4386, February 2006,
              <https://www.rfc-editor.org/info/rfc4386>.

   [RFC5026]  Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
              "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
              DOI 10.17487/RFC5026, October 2007,
              <https://www.rfc-editor.org/info/rfc5026>.
Top   ToC   RFC8552 - Page 13
   [RFC5509]  Loreto, S., "Internet Assigned Numbers Authority (IANA)
              Registration of Instant Messaging and Presence DNS SRV RRs
              for the Session Initiation Protocol (SIP)", RFC 5509,
              DOI 10.17487/RFC5509, April 2009,
              <https://www.rfc-editor.org/info/rfc5509>.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
              <https://www.rfc-editor.org/info/rfc5518>.

   [RFC6118]  Hoeneisen, B. and A. Mayrhofer, "Update of Legacy IANA
              Registrations of Enumservices", RFC 6118,
              DOI 10.17487/RFC6118, March 2011,
              <https://www.rfc-editor.org/info/rfc6118>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.
Top   ToC   RFC8552 - Page 14
   [RFC7553]  Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              DOI 10.17487/RFC7553, June 2015,
              <https://www.rfc-editor.org/info/rfc7553>.

   [RFC7566]  Goix, L. and K. Li, "Enumservice Registration for 'acct'
              URI", RFC 7566, DOI 10.17487/RFC7566, June 2015,
              <https://www.rfc-editor.org/info/rfc7566>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <https://www.rfc-editor.org/info/rfc7671>.

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/info/rfc7929>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8145]  Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
              Anchor Knowledge in DNS Security Extensions (DNSSEC)",
              RFC 8145, DOI 10.17487/RFC8145, April 2017,
              <https://www.rfc-editor.org/info/rfc8145>.

   [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
              Associate Certificates with Domain Names for S/MIME",
              RFC 8162, DOI 10.17487/RFC8162, May 2017,
              <https://www.rfc-editor.org/info/rfc8162>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8461]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
              <https://www.rfc-editor.org/info/rfc8461>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.
Top   ToC   RFC8552 - Page 15
6.2.  Informative References

   [RFC8553]  Crocker, D., "DNS Attrleaf Changes: Fixing Specifications
              That Use Underscored Node Names", RFC 8553,
              DOI 10.17487/RFC8553, March 2019,
              <https://www.rfc-editor.org/info/rfc8553>.

   [ZEROTOUCH]
              Watsen, K., Abrahamsson, M., and I. Farrer, "Secure Zero
              Touch Provisioning (SZTP)", Work in Progress, draft-ietf-
              netconf-zerotouch-29, January 2019.

Acknowledgements

   Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Martin Hoffmann,
   Paul Hoffman, Peter Koch, Olaf Kolkman, Murray Kucherawy, John
   Levine, Benno Overeinder, and Andrew Sullivan for diligent review of
   the (much) earlier draft versions.  For the later enhancements,
   thanks to Stephane Bortzmeyer, Alissa Cooper, Bob Harold, Joel
   Jaeggli, Benjamin Kaduk, Mirja Kuehlewind, Warren Kumari, John
   Levine, Benno Overeinder, Eric Rescorla, Adam Roach, Petr Spacek,
   Ondrej Sury, Paul Vixie, Tim Wicinski, and Paul Wouters.

   Special thanks to Ray Bellis for his persistent encouragement to
   continue this effort, as well as the suggestion for an essential
   simplification to the registration model.

Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086
   United States of America

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net/